Characterization and Simulation of High-Speed-Deformation-Processes

Abstract

The combination of the processes deep drawing and electromagnetic pulse forming is a promising way to cope with the ever higher complexity of new sheet metal designs. A cooperation between the Institute of Materials Science (IW) of the Leibniz Universität Hannover and the Institute of Applied Mechanics (IFAM) of the RWTH Aachen is investigating these processes both experimental and in simulation. Aim is the characterization of the combined process. Therefore the material properties of the investigated aluminum alloy EN AW 6082 T6 have to be determined quasi-static as well as at high speed. These properties are then used as a basic for the simulations. Anisotropic behaviors as well as dynamic hardening effects are investigated in the quasi-static state. Several experiments for analyzing "Bauschinger" respectively "Ratcheting effects" have been conducted resulting in a new measuring set-up for thin sheets. For the determination of high speed forming limit diagrams a novel testing device on the basis of the Nakajima-test has been developed allowing for strain rates of approximately 10^3 s^-1. Both testing methods are described in this paper; the results are then used to adapt the simulation models for the combined processes. The high speed deformation process is simulated by means of finite elements using a material model developed at the IFAM. The finite strain constitutive model combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamic setting. It is based on the multiplicative split of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong-Frederick kinematic hardening which is widely adopted as capable of representing the above metal hardening effects. To prevent locking of the simulated thin sheets a new eight-node solid-shell finite element based on reduced integration with hourglass stabilization developed at IFAM has been used. With these features it was possible to simulate the Bauschinger effect obtained by the previous experiments.